Delivery of a therapeutic agent from an implantable medical device can be desirable for a variety of applications. Therapeutic agents can be released from a medical device, such as an expandable stent or valve, to treat or mitigate undesirable conditions including restenosis, tumor formation or thrombosis. Procedures for mitigating certain conditions can include implantation of a device comprising a therapeutic agent. For example, the implantation of stents during angioplasty procedures has substantially advanced the treatment of occluded body vessels. Angioplasty procedures such as Percutaneous Transluminal Coronary Angioplasty (PTCA) can widen a narrowing or occlusion of a blood vessel by dilation with a balloon. Occasionally, angioplasty may be followed by an abrupt closure of the vessel or by a more gradual closure of the vessel, commonly known as restenosis. Acute closure may result from an elastic rebound of the vessel wall and/or by the deposition of blood platelets and fibrin along a damaged length of the newly opened blood vessel. In addition, restenosis may result from the natural healing reaction to the injury to the vessel wall (known as intimal hyperplasia), which can involve the migration and proliferation of medial smooth muscle cells that continues until the vessel is again occluded. To prevent such vessel occlusion, stents have been implanted within a body vessel. However, restenosis may still occur over the length of the stent and/or past the ends of the stent where the inward forces of the stenosis are unopposed. To reduce this problem, one or more therapeutic agents may be administered to the patient. For example, a therapeutic agent may be administered systemically, locally administered through a catheter positioned within the body vessel near the stent, or coated on the stent itself.
A medical device can be coated with a therapeutic agent in a manner suitable to expose tissue near the implantation site of the medical device to the therapeutic agent over a desired time interval, such as by releasing the therapeutic agent from an implanted stent into surrounding tissue inside a body vessel. Various approaches can be used to control the rate and dose of release of therapeutic agents from an implantable medical device. The design configuration of an implantable device can be adapted to influence the release of therapeutic from the device. A therapeutic agent can be included in the implantable medical device in various configurations. In some devices, the therapeutic agent is contained within an implantable frame or within a coating on the surface of the implantable frame. An implantable frame coating can include a bioabsorbable material mixed with a therapeutic agent, or coated over the therapeutic agent. Some implantable medical devices comprise an implantable frame with a porous biostable material mixed with or coated over a therapeutic agent. Implantable medical devices can also comprise a biostable material containing a dissolvable material and a therapeutic agent, where dissolution of the removable material upon implantation forms pores that release the therapeutic agent.
The design of a controlled release medical device can also depend on the desired mode of implantation of the device. The device can be adapted to the appropriate biological environment in which it is used. For example, a coated medical device for percutaneous transcatheter implantation can be sized and configured for implantation from the distal portion of a catheter, and adapted for expansion at the point of treatment within the body vessel by balloon or self-expansion. An implantable medical device can also be adapted to withstand a desired amount of flexion or impact, and should provide delivery of a therapeutic agent with a desired elution rate for a desired period of time.
Paclitaxel, and taxane analogues and derivatives thereof, can be used as a therapeutic agent coated on and released from implantable devices, such as stents, to mitigate or prevent restenosis. Paclitaxel is believed to disrupt mitosis (M-phase) by binding to tubulin to form abnormal mitotic spindles (i.e., a microtubule stabilizing agent). A therapeutic compound such as paclitaxel can crystallize as more than one distinct crystalline species (i.e., having a different arrangement of molecules in a solid form) or shift from one crystalline species to another. This phenomena is known as polymorphism, and the distinct species are known as polymorphs. Polymorphs can exhibit different optical properties, melting points, solubilities, chemical reactivities, dissolution rates, and different bioavailabilities. Paclitaxel and taxane derivatives thereof can be formed in an amorphous form, or in at least two different crystalline polymorphs. Solid forms of paclitaxel at room temperature include: amorphous paclitaxel (“aPTX”), dihydrate crystalline paclitaxel (“d PTX”) and anhydrous crystalline paclitaxel. These different solid forms of paclitaxel can be characterized and identified using various solid-state analytical tools, for example as described by Jeong Hoon Lee et al., “Preparation and Characterization of Solvent Induced Dihydrate, Anhydrous and Amorphous Paclitaxel,” Bull. Korean Chem. Soc. v. 22, no. 8, pp. 925-928 (2001), incorporated herein by reference in its entirety.
U.S. Pat. No. 6,858,644, filed Nov. 26, 2002 by Benigni et al. (“Benigni”), teaches a crystalline solvate comprising paclitaxel and a solvent selected from the group consisting of dimethylsulfoxide, N,N′-dimethylformamide, N,N′-dimethylacetamide, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, 1,3-dimethyl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone, and acetonitrile and combinations thereof. However, Benigni does not describe implantable device coatings comprising crystalline paclitaxel forms with different elution rates. Benigni discloses various solid forms of paclitaxel, including a first solid form reported as a highly water insoluble crystalline, granular, solvent-free form. The first solid form is substantially non-hygroscopic under normal laboratory conditions (relative humidity (RH) approximately 50-60%; 20-30° C.). However, when contacted with an atmosphere having a relative humidity greater than about 90%, or in aqueous suspensions, dispersions or emulsions, the first paclitaxel solid form reportedly converts (as a function of time, temperature, agitation, etc.) to a thermodynamically more stable second solid form. The second solid form is described as a trihydrate orthorhombic form having six water sites per two independent paclitaxel molecules (one paclitaxel “dimer”). These hydrated crystals reportedly present a fine, hair-like appearance and are even less water soluble than the first solid form. The second solid form is reportedly formed in aqueous suspensions or through crystallization from aqueous solvents in the presence of a large excess of water. This form is also disclosed in patent application EP 0 717 041, which describes the second solid form as being characterized by single crystal X-ray diffraction studies as being orthorhombic, with unit cells containing two crystallographically independent molecules of paclitaxel associated with hydrogen bonds to form a “dimer”. Mastropaolo, et al. disclosed a crystalline solvate of paclitaxel obtained by evaporation of solvent from a solution of Taxol® in dioxane, water and xylene. Proc. Natl. Acad. Sci. USA 92, 6920-24 (July, 1995). This solvate is indicated as being unstable, and, in any event, has not been shown to effect purification of crude paclitaxel. The thin plate-like crystals are reported to contain five water molecules and three dioxane molecules per two molecules of paclitaxel.
Many medical device coatings adapted for controlled release of taxane therapeutic agent such as paclitaxel rely on a polymer coating that is mixed with or applied above and/or beneath the releasable therapeutic agent to regulate the release of the therapeutic agent from the medical device surface. For example, U.S. Pat. No. 6,589,546 to Kamath et al. (filed Dec. 10, 2001) and Published US Patent Application 2004/0039441 by Rowland et al. (filed May 20, 2003) describe medical device coatings comprising a therapeutic agent mixed with a polymer to provide a controlled release of the therapeutic agent. Published US Patent Application 2003/0236513 by Schwarz et al. (filed Jun. 19, 2002) describes medical device coatings comprising a polymer coating deposited over or mixed with a therapeutic agent to control the rate of release of the therapeutic agent from the device.
What is needed are medical devices that permit controlled release of a therapeutic agent as a result of the solid form of the therapeutic agent, with or without a polymer. In particular, there remains a need for intravascularly-implantable medical devices capable of releasing a therapeutic agent at a desired rate and over a desired time period upon implantation. Preferably, an implanted medical device releases a therapeutic agent at the site of medical intervention to promote a therapeutically desirable outcome, such as mitigation of restenosis. There is also a need for a medical device with a coating of a releasable therapeutic agent coating having sufficient durability to resist the undesirable premature release of the therapeutic agent from the device prior to implantation at a point of treatment within a body vessel. In addition, there is a need for sufficiently durable medical device coatings comprising or consisting of a sustained-release taxane therapeutic agent while being free from a polymer or non-biocompatible organic solvents.